1. Field of the Invention
The present invention relates to an apparatus for detecting an open F number in a zoom lens camera.
2. Description of Related Art
In the zoom lens camera, a physical aperture diameter A is usually a constant value so that the open F number (i.e., the minimum F number) varies in accordance with the focal length f (F=f/A). The control of exposure, for instance a the open F number, and accordingly, the open F number must be input into an exposure control device to control the exposure. To this end, a conventional zoom lens having an automatic exposure function usually has means for detecting the open F number.
In a known open F number detecting device, a code plate of 4 bits is provided on the outer periphery of a cam ring which rotates to effect a zooming operation. The code plate is brought into contact with brushes to detect the angular position of the cam ring. This kind of detecting device, in which the angular position of the cam ring is detected by the code plate is advantageous in that the angular position can be detected as an absolute position only by the code plate bit data. However, such a known detecting device also has drawbacks, in that the manufacturing cost of the code plate is expensive, and in that detecting errors tend to occur because of failure in the electrical connection between the code plate and the brushes, if foreign matter enters therebetween. Furthermore, there is a possibility of chatter occuring between a conductive foil from which the code plate is manufactured and the brushes, resulting in the production of noise during zooming.